‘High Risk’ Severe Weather Days Are Rare And Dangerous

Few statements can steal a night’s sleep away from a meteorologist faster than the phrase “high risk for severe weather.” It’s rare to see one high risk issued during the course of a normal season, and we’ve now been through two separate high-risk severe weather events in just the past two weeks. Forecasters have to carefully decide when to use strong wording to grab the attention of folks who are in harm’s way. But some events appear so dangerous that forecasters have no choice but to go all-out to make sure people are aware and stay alert. 

The weather hasn’t been kind to the southeastern United States over the past couple of weeks. The region’s suffered through several different tornado outbreaks since March 17, which combined produced dozens of tornadoes and claimed several lives. The setup was different for each threat, but each day resulted in widespread damage.

A chart explaining the difference between each of the severe thunderstorm risk categories.

NOAA/NWS/SPC

Residents in the general area where each tornado outbreak occurred knew well in advance that they were at risk for bad weather. The Storm Prediction Center issues daily severe weather forecasts using a five-category scale that ranges from marginal risk (a 1/5) at the bottom of the scale to a high risk (5/5) at the top of the scale.

While the category names can seem a bit confusing and subjective if you’re not familiar with the scale—an enhanced risk (3/5) sounds worse to some folks than a moderate risk (4/5), for instance—overall, it’s a decent system to quickly relay the confidence and coverage of storms expected on any given day.

The very top of that scale, a high risk for severe weather, is seldom used. Forecasters reserve high risks for days when overwhelming evidence suggests that environmental conditions will support a major tornado outbreak or a derecho, a type of violent squall line that can produce significant wind damage over a large area.

It’s best to call a high risk the “break glass in case of emergency” option for the Storm Prediction Center. High risk days are so rare that there’s a Wikipedia page devoted to listing out every high risk issued in the past couple of decades. Before this month, the last scale-topping risk occurred in 2019, and there was another two-year gap between that high risk and the last one before it.

The SPC's severe weather outlook for Wednesday, March 17, 2021.

Forecast: NOAA/SPC | Map: Dennis Mersereau

Each of this month’s high risk days had a different setup and a different outcome. While the two days didn’t live up to their full potential given the environment in place, both days saw widespread damaging storms.

The March 17 tornado outbreak was the sixth-most prolific tornado event in Alabama history—producing 25 tornadoes there in a single day—while the outbreak on March 25-26 saw a handful of destructive tornadoes tear across parts of Alabama and Georgia, including an EF-4 tornado in Newnan, Georgia, and an EF-3 tornado south of Birmingham, Alabama.

The prospect of a high risk day is terrifying for folks in areas that routinely see severe weather during the spring. But it doesn’t take an ideal environmental setup with off-the-charts dynamics to create a storm that poses a grave threat.

High risks aren’t the end-all of significant severe weather threats. Some of the deadliest and most destructive tornadoes in recent history formed on days that didn’t see the kind of tornado outbreak that would require the use of a high risk. A tornado is a tornado no matter what, and it only takes one tornado hitting your location to make that day your own high risk day.

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